The present invention generally relates to semiconductor devices and more particularly to semiconductor light-emitting devices and laser diodes.
Particularly, the present invention relates to a laser diode operable in a wavelength range of 360-680 nm. Further, the present invention relates to a laser diode for use in optical recording and optical reading of information or light-emitting display of information. Further, the present invention relates to a semiconductor light-emitting device based on a III-V compound semiconductor material.
Further, the present invention relates to a vertical-cavity laser diode suitable for an optical source of optical recording and reading of information or light-emitting display of information. The present invention further relates to an optical information recording apparatus such as a xerographic image recording system or an optical system and optical telecommunication system including an optical interconnection device that uses a vertical-cavity laser diode.
In these days, efforts are being made to develop a red-wavelength laser diode operable in the wavelength range of 630-680 nm as an optical source of optical disk recording apparatuses. Such an optical disk recording apparatus includes a DVD (Digital Video Disk or Digital Versatile Disk) player. The laser diode is used in such disk recording apparatuses as the optical source for reading and/or writing of information.
In order to increase the writing speed of information into the optical disk in such optical disk devices, it is necessary to increase the output power of the laser diode used therein.
Hereinafter, a brief review will be made on conventional red-wavelength laser diodes.
FIG. 1 shows the cross-sectional diagram of a conventional red-wavelength laser diode of an AlGaInP system disclosed in the Japanese Laid-Open Patent Publication 11-26880.
Referring to FIG. 1, a substrate 1 of n-type GaAs carries thereon a buffer layer 2 of n-type GaAs, a cladding layer 3 n-type AlGaInP, a quantum well active layer 4 including therein alternate and repetitive stacking of an AlGaInP layer and a GaInP layer, a cladding layer 5 of AlGaInP of low carrier concentration (2-6×1017 cm−3), and an etching stopper layer 6 of p-type GaInP.
Further, there is provided a ridge structure 10 on a part of the etching stopper layer 6 wherein the ridge structure 10 includes a carrier-diffusion suppressing layer 7 of p-type AlGaInP, a cladding layer 8 of p-type AlGaInP, and a band-discontinuity relaxation layer 9 of p-type GaInP. Further, there are formed a pair of electric current blocking regions 11 of n-type GaAs on the surface part of the etching stopper layer 6 where the ridge structure 10 is not formed, and a contact layer 12 of p-type GaAs is formed continuously on the current blocking regions 11 and the band-discontinuity relaxation layer 9 therebetween. The contact layer 12 carries thereon a p-type electrode 13, and an n-type electrode 14 is formed on the bottom surface of the substrate 1.
In the laser diode of FIG. 1, there occurs a current confinement in the ridge structure 10 wherein the ridge structure 10 provides a current path between the current-blocking regions 11, and the electric current is confined into the ridge structure 10 thus formed of p-type GaAs. Further, it should be noted that the current-blocking regions 11 absorb the optical radiation from the quantum well active layer 4 and there is induced a refractive-index difference between the ridge structure 10 and the region outside the ridge structure 10 as a result of such an optical absorption. Thereby, there occurs an optical confinement in the ridge structure 10.
Such a ridge structure 10, while being able to form so-called optical loss-guide structure in the laser diode, has a drawback in that it increases the threshold current of laser oscillation due to the optical absorption caused by the current-blocking regions 10.
FIG. 2 shows the cross-sectional structure of a red-wavelength laser diode disclosed in the Japanese Laid-Open Patent Publication 9-172222.
Referring to FIG. 2, the laser diode is constructed on a substrate 15 of n-type GaAs and includes a buffer layer 16 of n-type GaAs, a cladding layer 17 of n-type AlGaInP, an active layer 18 of GaInP, a cladding layer 19 of p-type AlGaInP and an intermediate layer 20 of p-type GaInP, wherein the layers 16-20 are formed on the substrate 15 consecutively by an epitaxial process.
In the intermediate layer 20, there are formed a pair of stripe grooves reaching the p-type cladding layer 19, and the stripe grooves thus formed define a stripe ridge 21 therebetween. Further, current-blocking regions 22 are formed by filling the stripe grooves with a layer of n-type AlGaAs, and the entire structure is covered by a cap layer 23 of p-type GaAs formed by an epitaxial process.
In the case of the laser diode of FIG. 2, the current-blocking regions 22 are formed of AlGaAs having a bandgap larger than a bandgap of the active layer 18. For example, the current-blocking regions 22 are formed to contain Al with a concentration of 39% in terms of atomic percent when the laser diode is designed to operate at the wavelength of 650 nm. In the case the laser diode is to be operated at the wavelength of 630 nm, the Al content in the current-blocking regions 22 should be 45% or more in terms of atomic percent. In such a case, the current-blocking regions 22 are transparent to the laser beam and the loss at the optical waveguide is minimized.
FIG. 3 shows the cross-sectional diagram of a red-wavelength laser diode disclosed in the Japanese Laid-Open Patent Publication 7-249838.
Referring to FIG. 3, the laser diode is constructed on a substrate 24 of GaAs an includes, on the substrate 24, a cladding layer of n-type AlGaInP having a composition (Al0.6Ga0.4)0.5In0.5P, an active layer 26 having a quantum well structure formed by an AlGaInP barrier layer and a GaInP quantum well layer, an inner cladding layer 27 of p-type AlGaInP having a composition of (Al0.6Ga0.4)0.5In0.5P, an etching stopper layer 28 of p-type GaInP having a composition of Ga0.5In0.5P, an outer cladding layer 29 of p type AlGaInP having a composition (Al0.6Ga0.4)0.5In0.5P, a buffer layer 30 of p-type GaInP having a composition of Ga0.5In0.5P, and a cap layer 31 of p-type GaAs.
The laser diode is formed with a mesa structure by a wet etching process, wherein the wet etching process is conducted while using an SiN mask formed on the cap layer 31 with a width of 6 μm, until the etching stopper layer 28 is exposed. After the mesa structure is thus formed, a pair of current-blocking regions 32 of n-type AlInP and a pair of cap regions 33 of n-type GaAs are formed on the mesa surface. Thereby, the current-blocking regions 32 are grown so as to have a composition of Al0.5In0.5P on the part making contact with the mesa surface. After removing the SiN mask, a contact layer 34 of p-type GaAs is formed so as to cover the cap regions 33, the current-blocking regions 32 and the cap layer 31 on the mesa structure.
In the laser diode of FIG. 3, too, the problem of waveguide loss is avoided due to the large bandgap energy of AlInP used for the current-blocking regions 10. Further, the use of the AlInP current-blocking regions 32 is advantageous in view of the fact that AlInP has a smaller refractive-index as compared with the inner and outer cladding layers of p-type AlGaInP. Thereby, it should be noted that there is formed a real refractive-index difference between the region inside the ridge and the region outside the ridge, and a real refractive-index waveguide is formed in the laser diode.
In the laser diode of FIGS. 2 and 3, it should be noted that the current-blocking regions 22 or 32 contain an increased amount of Al for minimizing the optical absorption by the current-blocking regions. As noted already with reference to FIG. 2, the Al content in the current-blocking region 22 of AlGaAs has to be set to 39% or more in atomic percent when the laser diode is to be operated at the wavelength of 650 nm. In the case of the laser diode of FIG. 3, on the other hand, the current-blocking region 32 contains Al with an amount of in terms of atomic percent in the vicinity of the mesa surface, while this value of Al concentration is larger than the Al concentration (35% in atomic percent) of the AlGaInP cladding layer typically used in an AlGaInP laser diode. When the Al content in a semiconductor layer is large as such, there tends to occur a problem of optical damaging at the edge surface of the laser optical cavity due to non-optical recombination of carriers. It should be noted that the increase of Al content tends to increase surface states, while the surface states tend to facilitate the non-optical recombination of carriers.
Thus it is an object of the present invention to provide a red-wavelength laser diode having a reduced optical waveguide loss and simultaneously a reduced optical damage at the edge surface of the optical cavity formed in the laser diode.
As noted already, the laser diode of the AlGaInP system is becoming an important target of investigation in relation to application to laser beam printers, optical disk drives and the like, due to the fact that the laser diode of this system can produce an optical beam with the wavelength range of about 600 μm.
In the application to the optical source of disk drives, it is required that the fundamental mode of laser oscillation is a horizontal lateral mode of single peak. Further, it is required that astigmatism is small.
Such a single fundamental mode laser oscillation with reduced astigmatism is realized by using a real refractive-index waveguide structure, and there is proposed a visible-wavelength laser diode structure based on an AlGaInP system as represented in FIG. 4.
Referring to FIG. 4, the laser diode is constructed on a substrate 42 of n-type GaAs and includes a cladding layer 43 of AlGaInP, an active layer 44 of GaInP, and a cladding layer 45 of AlGaInP, formed consecutively on the substrate 42.
After forming the cladding layer 45, a ridge stripe of an inverse-mesa structure is formed so as to extend axially, and high-resistance regions 46 of AlInP are formed at both lateral sides of the stripe structure by causing a selective growth process while using an SiO2 mask on the stripe region. Further, a GaInP layer 48 and a p-type GaAs layer 49 are grown selectively and consecutively on the AlGaInP layer forming the stripe region While using an SiO2 mask formed on the high-resistance regions 46. Further, n-type GaAs regions 47 are formed on the high-resistance regions 46 at both lateral sides of the central stripe region, and a p-type electrode of Cr/Au/Pt/Au structure is formed on the top surface of the p-type GaAs layer 49. Further, an n-type electrode 41 of AuGe/Ni is formed on the bottom surface of the substrate 42.
In such a structure, there is formed a real refractive-index waveguide structure in correspondence to the central ridge stripe. Generally, such a laser diode is fabricated such that the epitaxial layers constituting the laser structure achieves a lattice fitting with the GaAs substrate 42.
On the other hand, the Japanese Laid-Open Patent Publication 5-41560 describes a refractive-index waveguide laser diode that uses a double heterostructure of a mixed crystal of (AlGa)aIn1-aP (0.51<a≦0.73) formed on a GaAs substrate, wherein the foregoing double heterostructure is formed with an intervening lattice misfit relaxation layer having a composition represented as GaPxAs1-x.
FIG. 5 shows the relationship between the band edge energy and the lattice constant for various III-V crystals, wherein the continuous lines represent the band edge energy of the conduction band Ec and the valence band Ev of a GaInP mixed crystal while the broken lines represent the conduction band energy and valence band energy of an AlInP mixed crystal.
Referring to FIG. 5, it can be seen that a mixed crystal of the AlGaInP system can be used for the cladding layer and the active layer as long as the AlGaInP mixed crystal has a composition in which the lattice constant is smaller than that of GaAs. When the composition is chosen as such, the bandgap energy increases and the laser oscillation wavelength shifts in the shorter wavelength direction. Thus, the foregoing Japanese Laid-Open Patent Publication 5-41560 proposes a laser diode that can oscillate at the wavelength shorter than 600 nm, by choosing the composition of the AlGaInP mixed crystal constituting the laser diode.
On the other hand, the relationship of FIG. 5 also indicates the possibility of improvement of performance of the red-wavelength laser diode oscillating in the wavelength range of 600-660 nm, by using a mixed crystal of AlGaInP having a lattice constant between those of GaAs and GaP, for the cladding layer and the optical waveguide layer.
Further, laser diodes having a refractive-index waveguide structure with current-blocking regions of GaAs or AlInP are proposed. In such a refractive-index waveguide laser diode, it is also possible to use a mixed crystal of AlGaInP for the current-blocking regions. However, the use of a mixed crystal composition containing a large amount of Al such as AlInP causes a problem to be described later.
In order to fabricate such a real refractive-index waveguide laser diode, it is necessary to form a real refractive-index profile in a transverse direction of the active layer. Normally, this is achieved by forming a ridge-stripe structure or a groove-stripe during the fabrication process of the laser diode by an etching process and by forming a cladding layer or current-blocking regions of AlGaInP, and the like, by a regrowth process.
In the case of forming a layer of AlGaInP on a substrate of GaP, GaAs or GaP0.4As0.6 by an MOCVD process, there is a tendency of extensive formation of hillock structure on the surface of the AlGaInP layer thus grown when the AlGaInP layer is grown on the substrate having a (100) principal surface or when the offset angle of the substrate principal surface from the (100) surface is small. This tendency of hillock formation is enhanced when the mixed crystal layer thus grown contains a large amount of Al as in the case of an AlInP mixed crystal.
It is possible to suppress the hillock formation to some extent by using an offset substrate and by increasing the offset angle of the substrate. However, such suppressing of hillock formation by way of using an offset substrate tends to become difficult in the case of an AlGaInP mixed crystal containing a large amount of Al and Ga and hence having a lattice constant smaller than that of GaAs. Further, use of an offset GaAsP substrate having a large offset angle poses a problem of availability as compared with the case of using a readily available industrial standard GaAs substrate.
When such hillock structure exists extensively in the semiconductor layers constituting a laser diode or an LED, the device performance or the yield of device production may be degraded seriously. This problem appears particularly serious in the case of regrowing a mixed crystal containing Al. In such a case, realization of a sufficient crystal quality is extremely difficult due to the surface oxidation of the underlying layer.
In the case of the laser diode disclosed in the Japanese Laid-Open Patent Publication 5-41560, op. cit., it is believed that fabrication of a satisfactory laser diode device with high-quality crystal layers is difficult.
Thus, it is an object of the present invention to provide a laser diode operable in the wavelength range of 600-660 nm wherein the device performance is improved by improving the quality of the crystal constituting the current-blocking regions.
A material of the AlGaInP system is a direct-transition type III-V material having the largest bandgap energy except for a material of the AlGaInN system. The bandgap energy can reach as much as 2.3 eV (540 nm in bandgap wavelength).
Thus, efforts have been made with regard to optical semiconductor devices of the AlGaInN system to provide a high-luminosity, green to red optical source for use in various color display devices or a laser diode for use in laser printers, compact disk drives, DVDs for optical writing of information.
In the case of a laser diode, a material system achieving a lattice matching with a GaAs substrate has conventionally been used. It should be noted that a laser diode for high-density optical recording is required to produce a large optical output of short-wavelength in a high temperature environment.
In order to construct a laser diode, it is necessary to provide a structure for confining both carriers and optical radiation in an active layer or light-emitting layer by using a cladding layer. Thus, a cladding layer is required to have a bandgap larger than a bandgap of the active layer.
In this regard, the material in the system of AlGaInP has a drawback in that the band discontinuity ΔEc on the conduction band tends to become smaller. In such a case, the injected carriers easily escape from the active layer into the cladding layer by causing an overflow. When such an overflow of carriers takes place, the threshold current of laser oscillation becomes sensitive with the operational temperature of the laser diode and the temperature characteristic of the laser diode is deteriorated.
In order to overcome the problem, the Japanese Laid-Open Patent Publication 4-114486 proposes a structure that uses an MQB (multiple quantum barrier) structure, in which a large number of extremely thin layers are stacked between the active layer and the cladding layer for carrier confinement. This structure, however, is complex, and it has been difficult to achieve the desired effect in view of the necessity of precision control of thickness of the layers to the degree of atomic layer level.
In an ordinary edge-emission type red-wavelength laser diode that uses a structure in which the active layer is sandwiched by a pair of optical guide layers having a composition represented as (AlxGa1-x)0.5In0.5P, the desired optical confinement is realized in the optical guide layers of the composition (AlxGa1-x)0.5In0.5P. On the other hand, the optical guide layers generally contain Al with a composition x of 0.5 or more, while such a high concentration of Al in the optical waveguide layer causes the problem of optical damaging at the optical cavity edge surface of the laser diode due to the recombination of carriers facilitated by Al. Thus, there has been a difficulty in obtaining a high optical output power or realizing a stable operation of the laser diode over a long period of time.
Summarizing above, conventional laser diodes constructed on a GaAs substrate with lattice matching therewith have a problem in operation under high temperature environment, or high-output operation, or operation over a long period of time. For example, it has been difficult to realize a red-wavelength laser diode operable under a high temperature environment such as 80° C. with high output power such as 70 mW or more, over a long period of time such as ten thousand hours. The difficulty increases with decreasing output wavelength of the laser diode.
The material of the system of AlGaInP having a lattice constant smaller than the lattice constant of GaAs is characterized by a wide bandgap and is suitable for decreasing the output wavelength of the laser diode or light-emitting diode. Thus, there is a proposal in the Japanese Laid-Open Patent Publication 8-18101 with regard to a light-emitting diode (LED) using the foregoing material system as well as other material systems. Further, there are proposals of a short wavelength laser diode oscillating at a wavelength of 600 nm or less. For example, the Japanese Laid-Open Patent Publication 5-41560 proposes a laser diode in which a double heterostructure having a composition of (AlGa)aIn1-aP (0.51<a≦0.73) and a lattice constant intermediate between GaAs and GaP is provided on a GaAs substrate with an intervening buffer layer of GaPxAs1-x having a composition adjusted so as to achieve a lattice matching with the foregoing double heterostructure. In the foregoing proposal, the problem of lattice misfit is resolved by interposing the buffer layer between the substrate and the double heterostructure.
FIG. 6 shows the relationship between the bandgap energy and the lattice constant for various III-V materials.
Referring to FIG. 6, the continuous lines represent the composition causing a direct-transition, while the broken lines represent the composition causing an indirect-transition. It should be noted that the material of the foregoing composition (AlGa)aIn1-aP (0.51<a≦0.73) having the lattice constant between GaAs and GaP falls in the region defined by the composition of AlInP and the composition of GaInP. By using the material system of AlGaInP having a bandgap larger than the bandgap of the material achieving a lattice matching with a GaAs substrate for the active layer and the cladding layers, it is possible to reduce the oscillation wavelength of the laser diode to be smaller than 600 nm.
FIG. 7 shows the construction of a laser diode having a refractive-index waveguide disclosed in the Japanese Laid-Open Patent Publication 5-41560, wherein the laser diode has a lattice constant between GaAs and GaP.
Referring to FIG. 7, the laser diode is constructed on a substrate 51 of n-type GaAs and includes a graded layer 52 of n-type GaPAs formed on the substrate 51, and a superlattice layer 53 of n-type Ga0.7In0.3P/(Al0.7Ga0.3)0.7In0.3P formed on the graded layer 52, wherein the substrate 51, the graded layer 52 and the superlattice layer 53 form together a GaPAs semiconductor substrate 54. The GaPAs semiconductor substrate 54 thus formed carries thereon consecutively an optical waveguide layer 55 of n-type AlGaInP having a composition of (Al0.7Ga0.3)0.7In0.3P, an active layer 56 of undoped GaInP having a composition of Ga0.7In0.3P, and an optical waveguide layer 57 of p-type AlGaInP having a composition of (Al0.7Ga0.3)0.7In0.3P, and a first buffer layer 58 of p-type GaInP, having a composition of Ga0.7In0.3P is provided further on the optical waveguide layer 57.
The first buffer layer 58 and the underlying optical waveguide layer 57 are then subjected to a mesa etching process to form a ridge stripe structure, wherein the mesa etching process is conducted such that the optical waveguide layer 57 is left with a thickness of 0.2-0.4 μm outside the ridge stripe structure.
At both lateral sides of the ridge stripe structure, a pair of current-blocking regions 59 of n-type GaInP having a composition of Ga0.7In0.3P are formed by a regrowth process, wherein the current-blocking regions 59 function also as an optical absorption region. Further, a contact layer 60 of p-type GaInP having a composition of Ga0.7In0.3P is formed on the current-blocking regions 59 including the ridge stripe region formed therebetween, by a regrowth process. Further, p-type electrode 62 and an n-type electrode 61 are formed respectively on the top surface of the contact layer 60 and on the bottom surface of the GaAs substrate 51.
In the foregoing laser diode that uses a material system having a lattice constant between GaP and GaAs, it is necessary to carry out three regrowth process steps, one for growing the GaInP buffer layer 58, one for growing the current-blocking regions 59, and one for growing the contact layer 60. Thereby, the fabrication process of the laser diode is complex and the yield of production tends to be reduced.
In order to facilitate the fabrication of a ridge-waveguide laser diode, there is also a proposal in the Japanese Laid-Open Patent Publication 10-4239, to form the current-blocking regions by way of oxidation of an AlGaAs mixed crystal having a composition represented as AlxGa1-xAs (0.8<x≦1). According to the foregoing proposal, the ridge structure is formed to have a width of 4 μm at the bottom part thereof, and there is provided a current path region as a non-oxidized part of the AlGaAs region of the foregoing composition of AlxGa1-xAs (0.8<x≦1), with a width of 3 μm.
According to the foregoing proposal, it is possible to form a laser diode having the current-blocking structure in a single crystal growth process.
On the other hand, the laser diode of the foregoing prior art has a drawback, in view of the difference in the lattice constant between the material system having a lattice constant between GaAs and GaP and the foregoing AlGaAs mixed crystal of the composition AlxGa1-xAs (0.8<x≦1), which achieves a lattice matching with the GaAs substrate, in that the thickness of the AlGaAs mixed crystal layer of the composition AlxGa1-xAs (0.8<x≦1) is inevitably limited when the AlGaAs mixed crystal layer is to be provided in the material system having a lattice constant between GaAs and GaP. Further, in view of the fact that the current path region of the not-oxidized AlxGa1-xAs (0.8<x≦1) mixed crystal layer extends such that the edge of the current path region is located near the edge of the ridge structure, there appears a substantial optical waveguide loss and increase of optical output power is difficult.
Thus, the present invention has an object to provide a semiconductor light-emitting device formed of a semiconductor material having a lattice constant between GaP and GaAs wherein the fabrication process is simplified. Further, the present invention has an object to provide a semiconductor light-emitting device formed of a semiconductor material having a lattice constant between GaP and GaAs wherein the optical waveguide loss is minimized and suitable for increasing output optical power.
Meanwhile, vertical-cavity laser diodes, which emit optical beam in a direction perpendicular to a substrate surface, draw attention in relation to application of red-wavelength optical source in the wavelength range of 630-650 nm for use in high-density optical disk drives and laser printers, in view of the fact that a vertical-cavity laser diode provides various advantageous features such as high-efficiency of laser oscillation, excellent beam property, excellent vertical mode property, and the like. Further, the vertical-cavity laser diodes are suitable for constructing a two dimensional array, and thus, there are possibility of application to the art of optical interconnection or optical array for laser beam printers.
In view of the limited length of optical cavity, a vertical-cavity laser diode requires to provide a large reflectance. Because of this reason, a distributed Bragg reflector (DBR) is generally used as the mirror of the vertical optical cavity. By using a DBR, it is possible to achieve a near 100% reflectance. A DBR is formed by stacking two semiconductor layers or dielectric layers having mutually different refractive index alternately and repeatedly with an optical distance corresponding to a quarter of the oscillation wavelength.
When the difference of refractive index between the two semiconductor layers constituting a DBR is large, a high reflectance is achieved with a reduced number of repetition. In order to avoid optical absorption and to increase the efficiency of laser oscillation, the semiconductor layers constituting the DBR are required to be transparent to the laser oscillation wavelength.
In the case of a vertical-cavity laser diode using the material of an AlGaInP system and oscillating at the wavelength of 630-650 nm, an active layer of GaInP is formed on a GaAs substrate, and a DBR is formed of high refractive layers of AlGaInP and low refractive layers of AlInP.
In view of the tendency of increase of bandgap and decrease of refractive index with increasing Al content in a semiconductor layer containing Al, it is desirable to construct a DBR by stacking AlInP layers and GaInP layers. Unfortunately, a GaInP layer is not transparent to the optical radiation in the wavelength range of 630-650 nm. Thus, there occurs a problem of optical absorption and degradation of optical cavity efficiency.
FIG. 8 shows the relationship between the lattice constant and bandgap for the GaInP and AlInP mixed crystals, wherein FIG. 8 shows the Γ valley energy and the X valley energy of the conduction band and further the band edge energy of the valence band. As can be seen from FIG. 8, the bandgap energy increases with decreasing lattice constant in the foregoing material system.
In the invention disclosed in the Japanese Laid-Open Patent Publication 9-199793, a DBR is constructed by combining an AlInP/GaInP layered structure formed on a GaAs substrate with a lattice constant smaller than the lattice constant of the substrate and an AlGaAs/GaAs layered structure, for reducing the optical loss caused by the DBR. According to the foregoing prior art, a first DBR structure of the AlGaAs/GaAs layered structure is formed on the GaAs semiconductor substrate and a second DBR structure of the GaInP/AlInP is formed thereon, with a graded layer interposed between the first and second DBR structures for relaxing the lattice misfit. On the DBR thus formed, a first graded cladding layer, a GaInP active layer and a second graded cladding layer are formed such that the composition grading is symmetric between the first and second graded cladding layers. Further, a further DBR structure is formed on the second cladding layer.
The invention disclosed in the foregoing Japanese Laid-Open Patent Publication 9-199793 is designed so as to minimize the optical absorption in the visible wavelength region and to improve the optical cavity efficiency. The two different material systems are used for constructing a DBR to eliminate the problem of lattice misfit of the AlGaInP mixed crystal and for avoiding the difficulty of growing a high quality AlGaInP mixed crystal layer. The difficulty of growing an AlGaInP layer will be explained later. Thus, the foregoing prior art uses the material system of AlGaInP for the DBR structure in the vicinity of the active layer where the intensity of optical radiation is large and uses the material system of AlGaAs for the DBR structure in the part away from the active layer in order to avoid the problem of degradation of the crystal quality associated with the increase of the number of stacks.
Further, there is another prior art vertical-cavity laser diode disclosed in the Japanese Laid-Open Patent Publication 10-200202 wherein the vertical-cavity laser diode of this prior art is constructed on a GaInP substrate.
According to this prior art, a substrate of GaInP having a composition of Ga0.75In0.25P is used and a DBR of the AlInP/GaInP is formed thereon with lattice matching. On the DBR thus formed, an active layer of GaInP is formed. According to this prior art, the problem of degradation of the crystal quality associated with lattice misfit is improved.
In the case of the forgoing prior art device of the Japanese Laid-Open Patent Publication 9-199793, it should be noted a plurality of DBR structures having different lattice constants are provided in a single laser diode device for changing the lattice constant. Further, in view of the fact that the DBR structure that causes a lattice misfit with the substrate has a large thickness, the use of the lattice misfit relaxation layer is not effective for improving the crystal quality. It should be noted that the DBR structure that causes a lattice misfit with the substrate contains at least 20 pairs of layers (40 layers or more) therein.
In the case of the laser diode disclosed in the Japanese Laid-Open Patent Publication 200202, a lattice matching is successfully achieved with respect to the GaInP layer transparent to the optical radiation in the wavelength range of 635-650 nm by choosing the lattice constant of the substrate to be smaller than the lattice constant of GaAs. On the other hand, the laser diode of the foregoing prior art has a drawback in that increase of Al or Ga content in the AlInP or GaInP material system facilitates hillock formation. Particularly, increase of Al content causes an extensive hillock formation and causes a serious problem in the AlInP material. There is no fundamental solution to this problem of hillock formation. When such defects are formed, the homogeneity of the heteroepitaxial interface is degraded substantially, and the optical scattering associated with such a poor quality interface increases the optical loss. Thereby, the optical cavity efficiency is deteriorated.
Further, the invention disclosed in the foregoing Japanese Laid-Open Patent Publication 10-200202 has a drawback, associated with the use of the GaInP active layer, in that there is a limitation imposed over the lattice constant when the laser diode is to be operated in the wavelength range of 630-650 nm.
More specifically, the wavelength of the GaInP mixed crystal that achieves lattice matching with GaAs is about 650 nm, and the wavelength becomes shorter when a GaInP mixed crystal having a lattice constant smaller than that of GaAs is used for the active layer. In order to achieve the foregoing desired wavelength range, it is therefore necessary to reduce the Ga content so as to increase the oscillation wavelength of the laser diode. However, such a decrease of the Ga content causes a compressive strain in the active layer and the quality of the crystal of the active layer is deteriorated. Thus, the lattice constant of the active layer is practically limited to the range close to the lattice constant of GaAs and the degree of freedom in designing the laser oscillation wavelength is limited.
On the other hand, the foregoing construction of the Japanese Laid-Open Patent Publication provides a possibility of increasing the degree of freedom in the laser diode design associated with the deviation of lattice constant from the lattice constant of GaAs, such as increased degree of freedom in selecting the material for various parts of the laser diode. It should be noted that the laser diode of the foregoing Japanese Laid-Open Patent Publication 10-200202 merely focuses on the problem of the optical absorption of the DBR, and no further proposals are made with regard to the improvement of other aspects of the laser diode.
There are further rooms for improvement in the vertical-cavity laser diode having a lattice constant between GaAs and GaP.
Thus, the present invention provides a vertical-cavity laser diode operable in the wavelength range of 630-660 nm and various optical systems using such a vertical-cavity laser diode.